In recent years, a transition from a mechanically-driven system to an electrically-driven system is on high demand, for aiming at an improvement of a space efficiency, assemblability, controllability and the like, especially required in vehicles. One of the examples of such transition is that a shift position switch mechanism for switching a shift position, or a gear position among P (Park) position, D (Drive) position and the like, in an automatic transmission of a vehicle is devised with a motor to drive the switching between those shift positions.
Such a shift position switch mechanism is equipped with an encoder that outputs a pulse signal at every predetermined angle in synchronization with rotation of the motor, and switches the power supply phases of the motor one by one based on a count value of the output signal of the encoder, to perform a rotational drive of the motor.
In such a system or a mechanism, diagnosis of encoder abnormality is performed, for example, by a technique disclosed in a patent document, JP 2013-72773 A (patent document 1). The technique disclosed in the patent document 1 is applied to an encoder that outputs a three-phase pulse signal, and, when the pulse signal is abnormal in one of the three phases, such an abnormality is picked up, i.e., is detected, as an abnormal pattern of the pulse signal combination, for the detection of the encoder abnormality.
However, the above-mentioned patent document 1 is silent about a noise handling measure for handling an overlapping noise on the output signal from the encoder, making the system prone to a false determination of the encoder abnormality when it is actually not the case, i.e., when the encoder is normally operating.